This invention relates to cold bonded pellets which contain a significant amount of an iron particulate material, which can be used as the feed material to a furnace for producing iron. The pellets are made from xe2x80x9ciron particulate materialxe2x80x9d, which, in addition to referring to iron ore powder as such, for example haematite powder, sinter feed fines, sinter fines, and ore concentrate fines, also includes a wide variety of particulate materials derived from iron smelting processes which contain significant amounts of recoverable ferrous values, such as electric arc furnace (EAF) dust, basic oxygen furnace (BOF) dust, mill scale, and the like. At present, the use of at least some of these materials in commercial iron smelting techniques presents significant problems.
Pellets containing particulate iron ore are used as a component of the feed in blast furnaces and DRI (direct reduced iron) furnaces. Commercially available iron ore pellets are formed by heat induration at temperatures of approximately 1300xc2x0 C. However, the heat-induration method has a number of drawbacks: the cost of production is high due to capital and maintenance expenditures on high temperature furnaces, and large amounts of energy are needed for the high temperatures used.
Several attempts have been made to use iron ore fines in DRI furnaces, such as the Iron Carbide Process, in which the particle size of the iron ore used must fall within a range suitable for bed fluidization, i.e. 0.1 mm to 1.0 mm, with a tolerance of about 5%; and processes using coal-bearing pellets such as the FASTMET process, in which the fines used must have a size range that is suitable for making strong green pellets, i.e.  less than 0.44 mm. However, these methods utilize only a narrow range of particle sizes, and therefore generate a significant amount of undersized waste. Several attempts have been made to develop a process for cold bonding iron ore pellets using either calcium silicate, limestone-silica, calcium carbonate, or organic glue as a binder, e.g. F.T.C. Doughty, xe2x80x9cOperation of a New Pelletizing Processxe2x80x9d, Iron and Steel International, December 1975; B. Hassler, xe2x80x9cControl of the Properties of Cold-Bound Autoclaved Pelletsxe2x80x9d, Stahl und Eisen 95 (1975) Nr. 16. However, cold bonded iron ore pellets have not been commercially successful because the binders used are not heat resistant, and the pellets disintegrate when heated above 700xc2x0 C., thus causing difficulties in the blast furnace environment.
It has also been proposed to use a Portland cement type binder to agglomerate iron ore powders using balling or briquetting techniques, for example in U.S. Pat. Nos. 3,264,090 and 4,846,884. In these methods, only a low amount of water is used, which appears to result in a product which is an outer shell bonded together by the Portland cement binder, with more or less loose unbonded powder inside. These products have the disadvantage that the Portland cement based binder is not able to withstand the temperatures of the blast furnace environment, with the result that above about 700xc2x0 C. the Portland cement bonds start to disintegrate and release iron ore powder into the gas flows in the furnace. An alternative complex process in which a binder comprising a mixture of styrene or acrylonitrile and vinyl polymers is used together with an aluminosilicate binder, such as Kayolite, and hydrochloric acid is described in U.S. Pat. No. 5,589,118.
Accordingly, it is desirable to develop an iron ore pellet product which can contain a broader range of iron ore particle sizes, and which also can contain particulate ferriferous materials other than ore powders, and in particular both EAF and BOF dusts.
Accordingly, this invention seeks to provide a cold bonded iron pellet product suitable for use in iron smelting procedures, including blast furnaces and DRI furnaces, which are rugged enough to withstand the furnace environment.
This invention in its broadest embodiment provides the product of a process in which an iron particulate material is mixed together with water and sufficient particulate high-alumina cement binder. The wetted mixture is then converted into shaped bodies such as pellets, briquettes, balls, and the like by conventional methods, or cast into moulds. After curing and drying, the shaped bodies can be fed to the furnace. Unlike at least some of the known processes for producing, for example, pellets, it appears that almost any iron particulate material of a suitable size can be used in the process of this invention. The amount of binder used is adjusted to provide a shaped body such as a pellet, briquette or ball with adequate strength to withstand the aggressive conditions which exist in an iron producing furnace environment, especially blast and DRI furnaces. The amount of water used is adjusted to suit the shaping procedure used, but is always at least enough to ensure complete hydration, and consequently complete setting, of the high-alumina cement binder. This ensures that the cement bond extends throughout the final shaped product, thus avoiding the formation of a cemented shell with more or less loose powder material trapped inside. Ideally, the shaped bodies after the cement has set are a coherent mass throughout. With proper choice of both the amount of water, and the amount of cement binder, cured pellets with adequate mechanical strength to survive a furnace environment which will smelt adequately to provide iron can be obtained.